This invention relates to an apparatus for reproducing information which has been recorded as a charge latent image.
There is a conventional apparatus for recording/reproducing information such as a graphic image in the form of a charge latent image as shown in FIGS. 1 and 2. FIGS. and 2 depict a recording system and a reproducing system, respectively.
Throughout the drawings, like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation.
First, the recording system will be explained with reference to FIG. 1. In FIG. 1, a recording medium 1 on which a charge latent image is intended to be recorded is composed of a charge holding layer 2 and an electrode 3. A photoelectric recording head 4 is composed of a photoconductive layer 5 and a transparent electrode 6.
The recording medium 1 and the photoelectric recording head 4 are arranged such that the charge holding layer 2 and the photoconductive layer 5 face each other across a predetermined space. The light carrying the optical image of an object O is allowed to be incident to the transparent electrode 6 of the photoelectric recording head 4 through an imaging lens 7, as depicted by an arrow A1.
A d.c. voltage source 9 is connected across the electrodes 3 and 6 through a switch 8. There will be a discharge generated across the charge holding layer 2 and the photoconductive layer 5 by turning on the switch 8.
In the configuration, the charge holding layer 2 is to hold charges for a long period of time and is formed of the material having extremely high insulation resistance, such as silicon resin. While, the photoconductive layer 5 is to generate pairs of an electron and a hole and is formed of such as amorphous silicon. In FIG. 1, the electrons are transferred to the transparent electrode 6 side in the photoconductive layer 5, on the other hand, the holes to the side in the layer facing the charge holding layer 2. The electrodes 3 and 6 are formed of a material such as ITO (Indium Tin Oxide).
The recording operation of the recording system constituted as above will be explained. As is depicted by the arrow A1, the light carrying the optical image of the object O is incident to the imaging lens 7 as depicted by the arrow A1 and is further incident to the photoconductive layer 8 of the photoelectric recording head 4, through the imaging lens 7 and the transparent electrode 6. The light is then absorbed in the photoconductive layer 5 to generate the electron-hole pairs therein.
Now the switch 8 is turned on to allow the voltage source 9 to apply a voltage across the electrodes 3 and 6. This causes the electrons in the photoconductive layer 5 being attracted by the positive polarity of the voltage source 9 to be transferred to the transparent electrode 6 side, while the holes to the side of the layer 5, facing the charge holding layer 2, as described above. The holes or the positive charge image corresponding to the optical image of the object O are therefore formed on the surface of the photoconductive layer 5, facing the charge holding layer 2.
Furthermore, there is a discharge generated across the photoconductive layer 5 and the charge holding layer due to the voltage applied by the voltage source 9. This discharge causes electrification on the surface of the charge holding layer 2 facing the photoconductive layer 5, so that charges Q are stored thereon.
The electrification due to the discharge depends on the distribution of the holes or the positive charge image on the surface of the photoconductive layer 5. The charge latent image corresponding to the optical image of the object O is thus transferred to the surface of the charge holding layer 2. Since the switch 9 causes the formation of the charge latent image, the switch 9 may be applied to a shutter of a camera.
Next, the reproducing system will be explained with reference to FIG. 2. In FIG. 2, there is arranged an optical reproducing head 10 facing the recording medium 1 on which the charge latent image has been recorded. The optical reproducing head 10 is composed of a photo-modulation layer 11 and a transparent electrode 12. The surface of the photo-modulation layer 11 faces the charge holding layer 2 of the recording medium 1, in the configuration. An optical reading unit 13 is further arranged to emit a reading light to the transparent electrode 12 of the optical reproducing head 10 as depicted by an arrow A2. The reading light passing the optical reproducing head 10 and the recording medium 1 is allowed to be incident to a photo-detection unit 14 as depicted by an arrow A3.
In the configuration, a photo-modulation layer 11 of the optical reproducing head 10 is formed of a material such as LiNbO.sub.3, BSO (Bi.sub.12 SiO.sub.20). liquid crystal, PLZT (Lead lanthanum zirconate titanate) or EC (Electrochromic), having photoelectric effect that light is varied accordingly with electric field. The photoelectric effect may be a double reflection effect, scattering effect or colouring effect. Selection of the material thus depends on the purpose. The optical reading unit 13 is composed of a laser light source, an analyzer etc. (not shown), to generate the light for reading the charge latent image. The photo-detection unit 14 is further composed of a wave-plate, an analyzer, a photoelectric convertor, etc. (not shown), to detect the light incident thereto.
The operation of the reproducing system will be explained. The charge latent image corresponding to the object O has already been recorded on the recording medium 1 as described with reference to FIG. 1. The optical reproducing head 10 is arranged adjacent to the recording medium 1 and the transparent electrodes 3 and 12 are electrically connected to each other. Under the configuration, electric field due to the latent charges Q will affect the photo modulation layer 11 to cause an electro-optic effect in the incident light.
Now the optical reading unit 13 emits the reading light to the optical reproducing head 10 to force the photo-modulation layer 11 to cause the electro-optic effect. The phase of the light emitted to the photo-modulation layer 11 and that of the light subjected to the electro-optic effect therein are therefore varied with respect to each other.
The reading light subjected to the electro-optic effect passes the recording medium 1 and is incident to the photo-detection unit 14. The intensity distribution of the reading light is varied accordingly with the distribution of the latent charges Q by the analyzer (not shown) and is converted to elecric siganls. As a result, the electric signals corresponding to the optical image of the object O are generated.
In the case of colour image photographing, the light from the object O is divided into primary colours by means of a proper optical equipment, to be recorded, reproduced and combined each other.
The conventional charge latent image recording/reproducing apparatus as described above has drawback that the intensity of the electric field due to the charge latent image, which affects the photo-modulation layer is gradually reduced as time lapses, because of the finite impedance of the photo-modulation layer of the reproducing system. This results in the reduction of gain, resolution, etc. during the reproduction.